The invention relates to an electrically controlled and preferably intermittently operating fuel injection system for internal combustion engines. The fuel injection system includes and electronic controller which determines the injected fuel quantity per unit time or per power stroke of the internal combustion engine and cooperates with an air flow rate meter which has a temperature-dependent resistor located in the induction tube of the engine. The resistor is located in one branch of a bridge circuit in a closed control loop and the heating current has a first DC component large enough to balance the bridge when the air flow rate Z is zero and an adjustable, superimposed second component. The second component has the value zero when Q is zero and can be increased with increasing air flow rate so as to compensate for the heat lost by the temperature-dependent resistor to the flowing induction air.
Known fuel injection systems include several injection valves, each valve associated with one cylinder of the internal combustion engine, wherein the temperature-dependent resistor located in the induction tube of the engine is heated by means of a heating coil, which transmits a constant amount of heat to the resistor either by radiation or convection. A resistor heated in this manner is subject to increased cooling when the aspirated air quantity increases and its electrical conductivity changes considerably. This change, which is a function of the aspirated air quantity, is used in the known system to control the opening duration of the one or several injection valves by making the resistor a part of a measuring bridge, the voltage across the diagonal branch of the bridge circuit increasing with increasing air flow. The diagonal voltage is used in the known system to control the time constant of a multivibrator which includes two mutually blocking transistors and the time constant of the multivibrator, in turn, determines the opening time of the valves.
When the temperature-dependent resistor is heated by a separate housing resistor, any changes in the heater supply voltage affect the precision of the air-flow measurement. Substantially higher precision may be achieved by heating the temperature-dependent resistor by an internal current and by adjusting the magnitude of this heating current via an electronic controller so that the operational temperature of the resistor remains practically constant. In that case, the magnitude of the heating current provides a reliable and precise source of information as to the time average of the aspirated air quantity.
The above described principle of hot-wire air-flow measurement is particularly suitable for fuel injection systems, because this constant temperature method permits the controlled heating current to follow the changes in the air flow rate within a response time of approximately 10 milliseconds or less. In the described process, the temperature-dependent resistor is one of the four elements in a resistance bridge in which the other three resistors are substantially temperature-independent and are located outside of the air flow. The two end-points of one of the diagonals of the bridge circuit are connected to the input of the controller and the controller delivers to the opposite bridge diagonal a heating current which changes with the aspirated air quantity. However, when using this process, two difficulties arise: When the internal combustion engine is stopped, the bridge must be balanced and the initial heating current must be such as to provide the necessary operating temperature for the temperature-dependent resistor. When the air through-put is maximum, i.e. during full load and maximum rpm operation of the engine, the heating current must be increased to two or three times its initial value. However, for control purposes, the interesting part of the information lies in the current increase and not in its initial value and the initial value should therefore be suppressed by subtraction. This results in a relatively low precision of the available information. The second difficulty resides in that the heating power N.sub.H from the resistor as a function of the time average of the aspirated air quantity Q obeys the relation EQU N.sub.H .alpha. [Q].sup.1/2
when the bridge equilibrium is established by changing the voltage across the opposite diagonal branch, the balancing signal is usually the voltage across the bridge resistor lying in series with the temperature-dependent resistor. Thus, the relation between the balancing voltage U.sub.S and the air throughput obeys the relation EQU U.sub.S .alpha. [Q].sup.1/4
this means that the useful signal U.sub.S changes only very little, even during substantial changes of the air throughput. For example, if the air flow rate changes in the ratio 1:35, the balancing voltage changes only by the ratio 1:2.5 which results in a low accuracy during the signal processing and during the adaptation of the injected fuel quantity to the aspirated air quantity.
In order to avoid these difficulties, a known fuel injection system provides that the heating current is composed of a first, steady DC component and a second component whose magnitude changes periodically. The latter component has the value 0 when the air flow rate Q is zero and it is increased with increasing air quantity until it produces enough heat to compensate for the heat lost to the air current.
In this system, the steady DC component is adjusted to the magnitude of the current needed when the air flow rate Q equals zero.
In this known system, the control current consists of heating current pulses, preferably of constant width, whose frequency is automatically adjusted by the controller and which are fed to a fuel control apparatus as air flow rate data.
When using a control process which affects the pulse frequency, it has been shown to be particularly favorable if the pulse duration is approximately 10 microseconds and if the pulse frequency is adjustable from approximately 1 kHz, preferably 2 kHz during idling, up to approximately 20 kHz and preferably up to 12 kHz during full-load and top rpm of the engine. Also provided is a pulse-width modulating system which prolongs the pulse duration with increasing pulse frequency and, for example, delivers pulses whose duration is increased from approximately 10 microseconds when the frequency is 2 kHz at idling, up to approximately 60 microseconds at a pulse frequency of 12 kHz at full-load and/or top rpm.
The available data regarding the aspirated air flow rate is presented in the form of a particular pulse frequency and this frequency must be associated with the opening duration of the injection valves associated with the individual cylinders of the internal combustion engine and hence with the injected fuel quantity to be delivered to the individual cylinders during each power cycle. For this purpose, the known fuel injection system includes an integrating stage whose charging circuit contains a storage capacitor fed by a constant current source which is turned on during the duration of the current pulse. The capacitor is charged in step-wise fashion during a predetermined crank shaft rotation, especially an angle of 180.degree.. The charge stored in the capacitor during a fixed rotational angle can later by transformed into a valve opening pulse by means of a constant current source, especially by a transistor adjusted for constant collector current and the valve opening pulse is adapted to correspond to the air quantity aspirated by an individual cylinder for each engine power cycle. The opening duration of the valves can be changed in a proportion of approximately 1:4 by prolonging the opening pulses in dependence on the input pulse frequency.